Life and Geologic Time
Fossils
• The remains, imprints, or traces of once living things
preserved in rocks.
• Fossils are usually found in sedimentary rock.
Why Fossils?
• Hard parts like teeth, bones, and shells are more
likely to become fossils.
• They are:
– less likely to be eaten by other organisms.
– slower in decaying.
– less likely to weather away.
• Rapid burial of an organism increases the chance of
being preserved.
Index Fossils
• Index fossils can be used to age or date rocks.
– Existed for a short period of geologic time.
– Large numbers.
– Geographically widespread.
– Easy to identify.
Geologic Time
• The appearance and disappearance of types of
organisms throughout Earth’s history give scientists
data to mark important changes or geologic
occurrences in time.
• Divide Earth’s history into smaller units based on the
types of life-forms living during certain periods.
• The division of Earth’s history into smaller units
makes up the geologic time scale.
• All the divisions in the geologic time scale are based
on changes in fossil organisms.
Geologic Time
• Record of Earth’s history, starting with Earth’s
formation about 4.6 billion years ago.
• Geologic time is divided into 3 subdivisions:
– Eras
– Periods
– Epochs
Eras
• The largest units of geologic time.
• There are four eras, all different lengths.
– Precambrian era – the longest, 4 billion years.
– Paleozoic era – “ancient life”
– Mesozoic era – “middle life”
– Cenozoic era – “recent life”
• Each era is determined by a change in life
forms.
Periods
• Eras are subdivided into periods.
• Vary in length and determined by life forms
and geologic events like mountain building.
Epochs
• Smallest units of geologic time.
• Only used in the Cenozoic Era’s periods where
evidence is more complete.
The Effect of Plate Tectonics:
• Plate tectonics is one process that causes
changing environments on Earth.
• If species adapt to the changes, or
evolve, they survive.
• If a species doesn’t have individuals with
characteristics needed to survive in the
changing environment, the species
becomes extinct.
Precambrian Era
• Longest geologic time unit of Earth’s history.
• 4.6 billion to about 540 million years ago.
• Relatively little is known about Earth and the
organisms that lived during this time.
– Precambrian rocks have been buried deeply and changed
by heat and pressure. They have also been eroded more
than younger rocks.
– Most fossils can’t withstand the metamorphic and
erosional processes that most Precambrian rocks have
undergone.
Life Forms of the Precambrian
• All life was marine (ocean dwelling) and was
soft bodied (no hard parts).
• The only plants were algae and fungi.
• Animal life included jellyfish, corals, and
worms (invertebrates – no backbones).
• Following the appearance of cyanobacteria,
oxygen became a major gas in Earth’s
atmosphere.